Washington -LRB- CNN -RRB- -- A mission to evacuate Americans from South Sudan was aborted Saturday when an aircraft carrying U.S. military members was fired upon as it prepared to land in Bor , wounding four of them , the Pentagon said .

The most severely damaged aircraft was thought to have been hit in the fuel line , a military official speaking on condition of anonymity said .

All three aircraft -- CV-22 Ospreys -- were diverted to Entebbe , Uganda , which is not where their flights originated , the official said . Another aircraft then flew the wounded to Nairobi , Kenya , U.S. Africa Command said in a statement .

The four service members were in stable condition after treatment , the statement said .

Pentagon officials were trying to determine how to mount another effort to evacuate the roughly three dozen Americans in South Sudan , where they have been working for the United Nations , a senior U.S. official said on condition of anonymity .

Secretary of Defense Chuck Hagel was `` reviewing options , '' Pentagon press secretary Rear Adm. John Kirby said . `` Whatever we do it will be in coordination with the State Department , '' he added .

The White House said U.S. President Barack Obama was briefed before dawn Saturday while aboard Air Force One after landing in Hawaii , then met with his national security team on the matter .

The fighting has displaced as many as 100,000 people , many of whom have crossed the Nile River , he said , adding that he feared a humanitarian disaster was unfolding .

South Sudanese President Salva Kiir blamed soldiers loyal to his former vice president , Riek Machar , for starting this month 's violence .

Tensions have been high in South Sudan since July , when Kiir dismissed Machar and the rest of the Cabinet . The move inflamed tensions between Kiir 's Dinka community and Machar 's Nuer community .

Casualties include soldiers and number in the hundreds , the government said .

U.S. Secretary of State John Kerry called Kiir on Saturday and discussed ways to halt the violence . It was Kerry 's second call to Kiir since Thursday night .

`` Secretary Kerry emphasized that only through leadership and political dialogue will the challenges facing South Sudan be resolved , '' State Department spokeswoman Jen Psaki said in a statement .

The two men `` discussed the need to prevent ethnic violence , their concern for the welfare of thousands of internally displaced persons fleeing the conflict , as well as for the safety of U.S. citizens in South Sudan , and they agreed to speak again soon , '' she said .

Meanwhile , the State Department issued an emergency message for U.S. citizens , calling on them to avoid the area around the airport in Bor . Limited flights were continuing from Juba International Airport .

On Friday , Kerry said he was sending a special envoy -- Ambassador Donald Booth -- to the country .

`` Now is the time for South Sudan 's leaders to rein in armed groups under their control , immediately cease attacks on civilians , and end the chain of retributive violence between different ethnic and political groups , '' Kerry said in a statement .

Saturday 's violence was n't the first this week to harm foreign troops in South Sudan . On Thursday , attackers killed two Indian army peacekeepers , wounded a third , and killed 20 of 30 civilians who were seeking refuge at the United Nations ' Akobo base , the U.N. said .

In a news release , the African Union called for an immediate truce .

It said that the chairwoman of the AU Commission , Nkosazana Dlamini-Zuma , was `` profoundly dismayed at the recent turn of events '' and condemned the killing of innocent civilians and U.N. peacekeepers in Bor as a war crime .

South Sudan became the world 's newest country when it split from Sudan in July 2011 . The split happened after a 2005 peace agreement ended years of civil war between the largely Animist and Christian south and the Muslim-dominated north .

The deal led to a January 2011 referendum in which people of the south voted to secede from Sudan .

CNN 's Nana Karikari-apau , Jason Hanna , Adam Aigner-Treworgy , Mading Ngor , Marie-Louise Gumuchian , Zain Verjee and Clare Hayes contributed to this report .

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NEW : African Union calls for an immediate truce

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Wounded service members in stable condition in Kenya , military official says

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U.S. troops were on mission to evacuate Americans from violence-hit city

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Pentagon trying to determine how to evacuate more than 30 Americans